Japanese Patent Provisional Publication No. 8-121124 discloses a conventional valve timing control device. Briefly explaining, in such conventional valve timing control devices, a vane rotor fixedly connected to the end of the camshaft, is rotatably accommodated in a cylindrical hollow housing of a timing pulley whose opening ends are closed by means of a front cover and a rear cover. A timing-advance-phase hydraulic chamber and a timing-retard-phase hydraulic chamber are defined between two opposing, substantially trapezoidal partition wall portions inwardly projecting in the diametrical direction and formed on the inner peripheral wall surface of the housing, and two vane portions of the vane rotor. The vane portions are properly rotated in their normal-rotation direction and reverse-rotation direction by a driving hydraulic pressure produced by way of proper pressure apply and release to each of the timing-advance-phase and timing-retard-phase hydraulic pressure chambers, depending on the engine operating conditions, and thus the relative phase angle of the camshaft to the timing pulley is variable. In this manner, the valve opening and closing timing of the intake valve can be variably adjusted.
As is generally known, the camshaft experiences positive (normal-rotation) and negative (reverse-rotation) torque fluctuations (alternating torque) owing to spring bias of the valve spring during operation of the engine. During rotation of the previously-noted vane rotor to the timing-advance phase or to the timing-retard phase, assuming that the camshaft experiences a great fluctuating torque, the driving hydraulic pressure for the vane rotor is overcome by reaction of the fluctuating torque and as a result the vane is pushed back. This gives the vane rotor an unstable rotational behavior. That is to say, when the vane rotor rotates to the timing-advance phase, the driving hydraulic pressure fed to the timing-advance-phase hydraulic pressure chamber is overcome by reaction of the positive fluctuating torque, and thus the vane rotor rotates toward the timing-advance phase, while repeating the normal rotation to the timing-advance phase and the reverse rotation to the timing-retard phase (timing advance and retardation) as indicated by the solid line of FIG. 9. In the same manner, the camshaft rotates relative to the timing pulley, while repeating normal rotation and reverse rotation. Thus, the responsiveness of the valve timing control corresponding to the opening and closing timing control for valves of the engine, would be lowered.
Especially within an engine operating region wherein the hydraulic pressure is low, it is desirable to enhance the responsiveness of the valve timing control device when switching from the timing-retard control to the timing-advance control.
According to the technique described in a Japanese Patent Provisional Publication No. 8-121123, a pilot-operated check valve, consisting of a check valve and a pilot valve, is provided in the vane. Both of the check valve and the pilot valve cooperate with each other to suppress back-flow of the driving hydraulic pressure supplied to the timing-advance-phase or timing-retard-phase hydraulic pressure chamber back to the oil passage, so as to prevent the reverse rotation of the vane due to the fluctuating torque.
However, in the latter prior art, the back-flow of the driving hydraulic pressure for each of the hydraulic pressure chambers is prevented by means of the pilot-operated check valve, and also the pilot-operated check valve is designed to directly utilize hydraulic pressure supplied to the respective hydraulic pressure chamber, and thus there is a possibility that the operating accuracy is also lowered owing to the decrease in holding ability of the hydraulic pressure in the respective hydraulic pressure chamber. That is to say, very small clearances are defined between the front cover and the front end of the vane, sliding and rotating within the housing, and defined between the rear cover and the rear end of the vane, in order to ensure better sliding-rotation performance of the vane. On the other hand, there is a great differential pressure between the adjacent hydraulic pressure chambers. For this reason, there is a risk of leak in hydraulic pressure supplied to one of the hydraulic pressure chambers via the very small clearance to the other hydraulic pressure chamber. As a result of this, there is a possibility that the checking function of the pilot-operated check valve is lowered and thus the undesired back flow cannot be effectively prevented.
Additionally, in the event that hydraulic-pressure leak occurs from the respective hydraulic pressure chambers to the exterior owing to degradation of each of seal members disposed between the housing and the front cover and between the housing and the rear cover, which may be degraded after a long period of time has elapsed, there is a possibility that the checking function of the pilot-operated check valve is lowered.
As a result of this, there is a technical problem similar to the former prior art, that is, the responsiveness of the valve timing control is lowered.